Patent GB-1117170-A (GB)
Title: Improvements in or relating to apparatus for determining a suitable threshold level with which to detect a wanted signal in the presence of noise
Inventor: HOUGHTON STANLEY JOHN
IPC: G06V10/28
Filed: 1966-12-15
Abstract: 1,117,170. Selecting threshold levels. MULLARD Ltd. 13 April, 1967 [15 Dec., 1966], No. 56208/66. Heading G4R. A threshold level for application to a signal is derived from the rate of change with respect to threshold level of the proportion of time the signal is above (or below) each of a series of threshold levels, being chosen at or near a minimum in the rate curve using absolute (modulus) values. In a character recognition system, a signal 24 from a flying-spot scanner 2 is digitized to two-state form in a threshold device 23, the threshold level of which is selected as follows. The signal to be digitized is applied in sampled form 1 to a series of threshold devices 3 having different regularly-spaced thresholds. Counters 4 each count the number of times the signal exceeds (or falls below) the threshold of a corresponding one of the devices 3, the counts being subtracted in pairs in subtractors 9 to obtain a measure of the slope of the curve of " proportion of time signal amplitude is above (or below) a given threshold " versus " threshold " at various threshold values. The differences are compared in pairs in comparators 10 which control AND gates 11, in part via inverters 12, so that one of the AND gates 11 is enabled to select the threshold level to be applied to the signal at 23. The ith AND gate 11 will be enabled if the outputs of both the ith and (i + 2)th subtractors 9 are greater than the output of the (i + 1)th subtractor 9, and will select a threshold between those of the (i + 1 )th and (i + 2)th threshold devices 3. Thus the selected threshold corresponds to a minimum in the slope of the curve mentioned, i.e. a point of inflexion. To prevent spurious outputs resulting from flat portions at the extremes of the curve and from an additional minimum likely to occur at a lower threshold level, the ith AND gate 11 can be inhibited if the (i + 2)th or (i + 3)th counter 4 or either holds zero, and (Fig. 3, not shown) a given AND gate 11 can be inhibited if any higher-numbered AND gate 11 produces an output. The AND gates 11 may select the threshold via further gates controlled by the outputs of further comparators (16, Fig. 4, not shown), the ith of which compares the outputs of the ith and (i + 2)th subtractors 9, so that either the (i+1)th or the (i+2)th threshold will be selected in response to enablement of the ith AND gate 11, whichever of the thresholds the slope minimum is closest to, determined on the assumption that the slope varies symmetrically about its minimum. As an alternative (Fig. 5, not shown) the threshold voltage determined by the AND gates 11 may be decremented by a correction voltage calculated from the outputs of the ith, (i+1)th and (i+2)th subtracters 9 (assuming the ith AND gate 11 gives an output) on the assumption that the slope m satisfies where x is threshold, b is the threshold at the slope minimum, and a, c are constants. As a modification, the subtracting and counting operations in Fig. 2 could be interchanged, the ith and (i+1)th threshold devices 3 feeding an ith further AND gate, one via an inverter, and this AND gate feeding an ith accumulator, the counters 4 and subtractors 9 being eliminated. The parallel mode operation with respect to thresholding at 3, etc., could be replaced by serial mode.